This invention relates generally to tubular ceramic articles and, more particularly, to tubular ceramic articles comprised of silicon and silicon carbide.
Radiant burner tubes are used in high temperature, corrosive environments such as found in gas fired industrial heat treating furnaces and aluminum melting furnaces. Commercially available radiant burner tubes include metal alloy (e.g., nickel-chromium-based alloy) tubes, ceramic composite (e.g., oxide ceramic fibers in a silicon carbide ceramic matrix) tubes, and ceramic (e.g., silicon carbide) monolith tubes. Such tubes typically have an upper use temperature in a range of about 1100.degree. C. (2012.degree. F.) for metal alloy tubes, to about 1260.degree. C. (2300.degree. F.) for ceramic composite tubes to about 1350.degree. C. (2462.degree. F.) for silicon carbide monolithic tubes.
Although monolithic silicon carbide radiant burner tubes are available, such tubes are typically very expensive.
Through the taking of great care and by making some compromises in properties it may be possible to select a ceramic composite from which to prepare a radiant burner tube which generally meets most, but not necessarily all, of the requirements for use in high temperature, chemically corrosive environments.
Silicon carbide, a crystalline compound of silicon and carbon, has long been known for its hardness, strength, and generally excellent resistance to oxidation and corrosion. Silicon carbide has a low coefficient of expansion, good heat transfer properties and exhibits high strength and excellent creep resistance at elevated temperatures. These desirable properties may be attributed to strong covalent bonding. However, such strong bonding may also be the cause of a generally undesirable property of silicon carbide of being difficult to work or fabricate into particular useful shapes. For example, because at high temperatures silicon carbide dissociates rather than melts, it is not generally feasible to fabricate articles from silicon carbide by melt processing. Also, because silicon carbide has a relatively very slow diffusion rate, fabrication of such articles by plastic deformation processes is generally not viable.
It has been proposed to produce shaped silicon carbide articles by forming bodies of silicon carbide particles and either bonding or sintering the particles at high temperatures to form a consolidated body. If the particulate silicon carbide starting material is fine enough, and suitable sintering aids are added, the fine, particulate material will exhibit sufficient self-diffusion at high temperatures that the particulate material will sinter and form into a substantially dense single phase material. Sintering processes, in general, require fine powder starting materials and pressureless sintering processes, in particular, require an even finer starting material. Because of the needed fineness and high purity of the starting materials, articles formed by sintering processes are relatively expensive.
Coarser and less pure silicon carbide powders are known to bond together at high temperatures. However, the resultant products have considerable porosity and for that reason are usually not as strong, or as corrosion resistant, as more fully densified materials. The properties of such materials may be substantially improved by infiltrating the pores of such materials with silicon, in either vapor or liquid form, to produce a two phase, silicon-silicon carbide product. Although such processes utilize relatively inexpensive coarse powders as starting materials, they require two high temperature treatments such as in a furnace, one to form the silicon carbide to silicon carbide bond and a second, separate high temperature treatment, to infiltrate the formed body with silicon.
Mixtures of coarser and less pure silicon carbide powders with particulate carbon or with a carbon source material may be preformed and subsequently impregnated with silicon at high temperature to form "reaction bonded" or "reaction sintered" silicon carbide products. The carbon component may be in the form of particulate graphite or amorphous carbon, or may be in the form of a carbon source material, for example a carbonizable organic material, such as pitch, resin or similar materials, which will decompose during furnacing to yield carbon. The infiltrating silicon reacts with the carbon in the preformed body to form additional silicon carbide which bonds with the original silicon carbide particles to produce a dense silicon carbide article. Typically reaction bonded silicon carbide materials are characterized by almost zero porosity and the presence of a second phase, or residual, of silicon, usually greater than about 8% by volume.
In typical siliciding or typical reaction bonding processes, the particulate silicon carbide and carbon starting material is initially preformed or preshaped into an article, commonly referred to as a "green body", which is subsequently fired. The particulate silicon carbide and carbon starting mixture is commonly blended with a binder to aid in shaping. If the binder is dry, or relatively dry, the powder may be compacted to the desired shaped green body such as by using a press. If the binder is liquid, or semi-liquid, and is used in sufficient quantity, the mixture may suitably be slip cast, extruded or injection molded to form a shaped green body.
High temperature heat exchange components desirably have relatively thin walls to facilitate high rates of heat transfer. In general, previous techniques for fabricating tubular articles of silicon carbide have generally met with varying degrees of success.
One previously disclosed technique is the subject of commonly assigned Kasprzyk, U.S. Pat. No. 4,789,506, issued Dec. 6, 1988. In accordance therewith, a tubular article of silicon carbide and silicon is produced by concentrically positioning a vertical tubular column particulate silicon contiguous to a hollow, vertical tubular column of particulate silicon carbide, carbon, or mixtures of silicon and carbon, and heating the adjacent columns to a siliciding temperature. The silicon component infiltrates the column containing the particulate silicon carbide, carbon, or mixtures thereof, to form the tubular article.
While such a technique generally works well in producing straight tubular articles, such a technique is generally not well suited for producing arcuate tubular ceramic articles or ceramic tubular articles having arcuate sections or portions.
Consequently, arcuate tubular ceramic articles are generally formed by an alternative technique, such as by slip casting, for example. Unfortunately, such processing can be relatively slow and may also be either or both equipment and labor intensive. For example, such processing typically requires the use of various mixing and molding equipment and the labor associated therewith. As will be appreciated, such processing generally requires a large supply of plaster molds to permit the fabrication of a particular article in typical commercial production quantities. Further, such processing typically requires the use of various pieces of equipment such as mixing mills, vacuum deairing chambers and drying ovens as well as correspondingly large areas of floor space. Consequently, such processing can be more costly and time consuming than is desired or preferred.
In the past, mandrels have been used to make tubular articles. Rigid tubular articles have been made by placing the material that will form the tubular article about the mandrel and then hardening the material to a rigid state. After the tubular article has become rigid, the mandrel is removed from the interior thereof.
The removal of a mandrel from a tubular article having a simple shape is generally easily accomplished by sliding the mandrel out from the interior of the tubular article. For instance, for a tubular article having a straight, as opposed to a curved or bent shape, the corresponding mandrel is generally easily removed by simply sliding the mandrel out from the interior of the tubular article.
Conventional mandrels come in one piece and have an outer shape that generally corresponds to the shape of the interior of the tubular article being produced. In the past, it has been difficult to form rigid tubular articles in more complex forms, such as forms including both a straight portion and an arcuate portion, through the use of a single conventional mandrel. In practice, the fabrication of such tubular articles has necessitated the use of more than one mandrel: one mandrel is used to form the straight portion and a second mandrel is used to form the arcuate portion. These individual tubular segments formed from each of the separate mandrels are subsequently joined together to form the desired final tubular product.